There have been numerous prior art hybrid cicruits employing dual detector linearized opto-isolators. These prior art hybrid circuits include U.S. Pat. No.
4,056,719 issued Nov. 1, 1977 to S. G. Waaben. The Waaben patent teaches an isolation circuit which provides the required separation by converting a two-wire termination to a four-wire circuit and then back again to a second two-wire termination. Each leg of the Waaben four-wire circuit employs one of a pair of dual detector opto-isolator circuits for unidirectional transmission and isolation while a signal cancellation scheme is also provided to prevent feedback. This opto-isolator and signal cancellation combination eliminates the need for line transformers to provide the required isolation.
The Waaben dual detector opto-isolator circuits achieve a linear transmission characteristic by providing feedback through an auxiliary detector to the input of the opto-isolator thereby compensating for the inherent nonlinearity of the opto-isolator device. The Waaben two-wire to two-wire isolation circuit uses one of a pair of these linearized opto-isolator circuits to transmit signals from the tip and ring conductors of the first two-wire circuit through a first "transmit" section to the second two-wire circuit while the other dual detector opto-isolator circuit transmits signals from the second two-wire circuit through a second "transmit" section to the tip and ring conductors of the first two-wire circuit. However, since both transmitted and received signals appear together on the tip and ring conductors of both two-wire circuits, this necessitates the use of signal cancellation impedances and summing amplifiers to prevent the signals transmitted by the first "transmit" portion from being fed back through the two-wire circuit to the associated "receive" portion and vice versa. The problem with the use of a signal cancellation impedance is that it, in turn, necessitates the use of a termination impedance in each two-wire circuit to match the input impedance of the isolation circuit to the impedance of the tip and ring conductors.